US airlines began canceling hundreds of flights Thursday due to the Federal Aviation Administration’s order to reduce traffic at the country’s busiest airports starting Friday because of the government shutdown.

More than 790 planned Friday flights were cut from airline schedules, according to FlightAware, a website that tracks flight disruptions. That number, already four times higher than Thursday's daily total, was likely to keep climbing.

The 40 airports selected by the FAA span more than two dozen states and include hubs such as Atlanta, Dallas, Denver, Los Angeles and Charlotte, North Carolina, according to the agency's order, which was published Thursday evening. In some metropolitan areas, including New York, Houston, Chicago and Washington, multiple airports will be impacted.

The FAA said in the order that the reductions will start Friday at 4 percent and ramp up to 10 percent by Nov. 14. They are to be in effect between 6 a.m. to 10 p.m. local time and impact all commercial airlines.

The decision to reduce service at “high-volume” markets is meant to maintain travel safety as air traffic controllers exhibit signs of strain during the shutdown. It also comes as the Trump administration is ramping up pressure on Democrats in Congress to end the shutdown.

“With continued delays and unpredictable staffing shortages, which are driving fatigue, risk is further increasing, and the FAA is concerned with the system’s ability to maintain the current volume of operations,” the order reads.

Hours before the reductions went into place, airlines were scrambling to figure out where to cut. American Airlines said it reduced its schedule at the listed airports by 4 percent from Friday through Monday, about 220 cancellations each day, and would move from there toward the 10 percent target. The carrier said its international schedule was expected to remain untouched.

Passengers with plans for the weekend and beyond waited nervously to see if their flights would take off as scheduled. Some travelers began changing or canceling itineraries preemptively.

Flight Cuts May Impact Smaller Carriers
The restrictions also apply to a subset of smaller carriers that operate scheduled charter flights. International flights do not have to be reduced, according to the FAA.

Some airlines planned to focus on slashing routes to and from small and medium-sized cities.

“This is going to have a noticeable impact across the US air transportation system,” industry analyst Henry Harteveldt said.

The flight reductions just weeks before the busy holiday season prompted some travelers to change their plans or look at other options.

Fallon Carter canceled her Friday flight from New York to Tampa, Florida, where she planned to spend the weekend at the beach. She was worried about making it back to Long Island to be a bridesmaid at her best friend's wedding.

“I don’t know if I get there, will I get home?” Carter said.

The FAA is imposing the reductions to relieve pressure on air traffic controllers who are working without pay during the shutdown, which began Oct. 1, and have been increasingly taking sick days. Most controllers work mandatory overtime six days a week, leaving little time for side jobs to help cover bills unless they call out.

In recent weeks, the FAA has delayed flights when airports or its other facilities are short on controllers.

Shuffling Schedules
Airlines said they would try to minimize the impact on customers, some of whom will see weekend travel plans disrupted with little notice.

The airlines will be required to issue full refunds but not to cover secondary costs such as food and hotel accommodations unless a delay or cancellation results from a contributing factor that is within the control of the airlines, according to the Department of Transportation.

The head of Frontier Airlines recommended that travelers buy backup tickets with another airline to avoid being stranded.

The cuts also could disrupt package deliveries because two airports with major distribution centers are on the list -- FedEx operates at the airport in Memphis, Tennessee, and UPS in Louisville, Kentucky, the site of this week’s deadly cargo plane crash.

The FAA also said commercial space launches will only be allowed between 10 p.m. and 6 a.m. starting Monday. That could force SpaceX to reschedule some upcoming planned afternoon launches.

The cuts could affect as many as 1,800 flights, or upward of 268,000 passengers, per day, according to an estimate from Cirium.

Airlines are used to dealing with canceling thousands of flights on short notice during severe weather, but the difference now is that these cuts during the shutdown will last indefinitely until safety data improves.

Shutdown Already Straining Travel
The shutdown is putting unnecessary strain on the system and damaging confidence in the US air travel experience, said US Travel Association President and CEO Geoff Freeman.

Kelly Matthews, who lives in Flat Rock, Michigan, and flies every week, said she canceled most of her upcoming trips and understands why federal airport employees have stopped showing up.

“You can’t expect people to go in to work when they’re not getting a paycheck for the continuation of over a month now,” she said. “I mean it’s not a matter of them not wanting to do the job -- but you can’t afford to pay for gas, your day care and everything else.”

The past weekend brought some of the worst staffing issues since the start of the shutdown.

From Friday to Sunday evening, at least 39 air traffic control facilities reported potential staffing limits, according to an AP analysis of operations plans shared through the Air Traffic Control System Command Center system. The figure, which is likely an undercount, was well above the average for weekends before the shutdown.